U. h. f. low noise amplifier



Dec. 24,` 1957 R. P. DECKER u. H. F. Low NOISE AMPLIFIER 3 SheeItS-Sheet 1 Filed Dec. 30, 1955 INYENTOI?4 Rnmsnvl? Decken?v A rronusvs Dec. 24, 1957 R. P. DECKER u. H. F. Low NoIsE AMPLIFIER s Sheets-shea z Filed Dec. 30, 1955 N uil mw m9 v km.

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U. H. F. LOW NOISE AMPLIFIER Filed Dec. 30, 1955 3 Sheets-Sheet 3 G R 9' t f NOISE: 8 y F (ggfs 6 Gt REIQ'/ l, @t Req@ 2 i,

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rronnevs 'United States Patent U. H. F. LOW NOISE AMPLIFIER Ramsay l. Decker, Cedar Rapids, Iowa, assgnor to Colline Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application December 30, 1955, Serial No. 556,681

13 Claims. (Cl. 179-171) This invention relates to an ultra-high-frequency amplifier which is capable of having a very low noise-figure and can have narrow-band selectivity.

The noise-figure of a particular amplier is dened as its input signal-to-noise power ratio divided by its output signal-to-noise power ratio. Therefore, the minimum and best noise-figure that an amplifier can have is one; since, in such case, the output signal-to-noise ratio is the same as the input signal-to-noise ratio, and hence is not increased by noise internally generated in the amplifier.

The invention concerns a particular transmission-line arrangement which provides mismatching for the best noise-figure of an amplifier, having a grounded-cathode stage followed by a grounded-grid stage. A mismatching line of strip construction is provided by the invention, and can obtain the critical mismatching requirements at ultrahigh frequencies with relative ease, while permitting very little transmission power loss.

Conventionally, lumped-constant amplifiers, having a ground-cathode stage followed by a grounded-grid stage, are used in the VHF region below about 300 megacycles. However, such amplifiers have been considered impractical for use at frequencies above about 300 megacycles because of a number of factors, such as: their increasing instability as the frequency is increased, difficulties in neutralizing interelectrode capacitances, and difficulties in obtaining cancellation of stray susceptances at the higher frequencies.

Accordingly, in the ultra-high frequencies, groundedgrid amplifiers have been almost solely used primarily because they are easier to stabilize at ultra-high frequencies, and, therefore, are less likely to have parasitic regeneration than previously used grounded-cathode amplifiers.

However, this invention provides an amplier having a grounded-cathode first stage; which, at U. H. F. frequencies such as 1000 megacycles-per-second, can provide a noise-figure that is lower than the noise-gure provided by the same tube when connected as a conventional grounded-grid circuit and can be, for example, approximately 5.9 decibels, and is stable at U. H. F. frequencies.

This inventionlovercomes the previous U. H. F. disadvantages of ground-cathode amplifiers, and permits basic differences between the grounded-cathode circuit and the grounded-grid circuit to be utilized at ultra-high frequencies. One basic difference is that the former has a much higher input impedance than the latter. This permits the grounded-cathode amplifier to have better selectivity than the grounded-grid amplifier, if means can be provided to overcome difficulties inherent in groundedcathode operation at ultra-high frequencies, which is accomplished by this invention. Thus, for example, this invention can provide grounded-cathode arrangements that obtain an input bandwidth that may be a fraction of the bandwidth permitted by comparable grounded-grid arrangements.

Another basic advantage of the grounded-cathode circuit over the grounded-grid circuit is that the former is icc capable of higher actual gain. in the past, nevertheless, the gain advantages of the grounded-cathode have not been able to be realized in practice because of the instability that accompanied increased gain. However, this invention permits, at ultra-high frequencies, a groundedcathode stage to be loaded by the low impedance input of a grounded-grid stage, thus obtaining a combination ofy gain and stability heretofore unobtainable at ultrahigh frequencies.

Another basic advantage of the grounded-cathode amplifier over the grounded-grid amplifier is that the noisefigure of the latter is critically dependent on its output circuit losses, which is not the case with the groundedcathode circuit. This invention also permits this advantage to be realized at ultra-high frequencies.

The invention can obtain the best noise-figure available for a given type of amplifier tube; but it is generally desired that the tube chosen for the grounded-cathode stage be capable of providing a minimum noise-igure In general, these tube characteristics are: (1) generation of a minimum amount of internal noise, and (2) a minimum overall input conductance, which includes transittime loading effects. One suitable tube type is generally designated in the electronics industry as a 6280/416B planar-type triode.

The second tube in the invention, which provides a grounded-grid stage, may have a somewhat higher level of internal noise without substantially increasing the noise-figure of the amplifier and might, for example, be a pencil triode.

The primary object of this invention is, therefore, to provide an amplifier that has a noise-figure in the ultrahigh frequency region that is superior to that obtainable by presently known amplifiersf It is another object of this invention to provide a lownoise amplifier for the U. H. F. region that is relatively easy to construct and adjust.

It is still another object of this invention to provide a low-noise amplifier for the U. H. F. region that is capable of having much better input selectivity than conventional grounded-grid amplifiers operated in the same frequency region.

The invention features strip-type of transmission lines as the interconnecting and tuning medium of the amplifier, wherein a common ground-plane serves as one side of each line and a single conductor serves as the other side.

The characteristic impedance of the transmission lines in the invention should be closely controlled. This can easily be done by adjusting the spacing and/or dielectric material used with the strip-type of line. Since one side of the line is a common ground-plane, only a single strip of metal, comprising the other side, need be manipulated.

The invention provides mismatching features for some of its transmission lines utilizing susceptance-cancelling tuning stubs. A first line is between the amplifier input terminals and the input to the grounded-cathode stage and has a stub at each end. The line and tuning stubs have a predetermined characteristic impedance chosen, as will be described below, to permit the input of the groundedcathode stage to see the mismatch which provides an optimum noise-figure.

In a like manner, another line having susceptance-cancelling stubs at both ends is provided between the output of the grounded-cathode stage and the input to the grounded-grid stage. By proper choice 0f transmission line parameters and stubs, as taught herein, the groundedcathode stage can see an output impedance which provides stable operation and the grounded-grid stage can see substantially an optimum source impedance, which provides optimum noise characteristics.

Another stub is provided between the grid and plate electrodes of the grounded-cathode stage to neutralize its grid-to-plate interelectrode capacitance; and a stub is provided at the output of the grounded-,grid st age to match (or mismatch, depending on the particular case) it tol'a load, whichiis connected to `the output of the 1nvention.

Further advantages, lobjects and features of this 1nvention will be apparent to a person skilled in the art upon further study of'this speciiication and'drawings, 1n which:

" Figure 1 is a perspective view with several cut-away portions io illustrate ooo forro of iho iii-volition;

Figiifo 2 iS o' Sooiioaol View ialioo along Plagio. 2f/2f- 2,-2 in Figure l; 'i Figure 3 provides a set of curves that illustrate the folaiiooohio boiwooo ioolll'iioooioooo' mistilatoli ooi ,oooplier noise-figure; and, v

Figure '4 is' nood to explain f roosoiiooiooflioo ,ilioory utilized in this invention. i i Now referring to the invention in more detail, Figure l shows the yinvention assembled about a metallic groundplane lll. A first opening v411 is lformed in ground-plane 10; and a tube 12, which might fer example, a planar type, designated in the industry as 4l6l,'is supperted in opening 11 by a tube support 13, best shown y1n Figure 2, "Tube support 13 includes a cylindrical-shaped well 14, made of insulating material, which has ilange that may be fastened to growl-Plano l by moelloof bolts, for example.

Aplurali'ty of contaet fingers 17 made of a concluating material are supported inside of well 13, and project inwardly to contact lthe Vcathode-contacftin'g surfaee 1S of tubo 12- A tubo sooloot 19, similar to maar common available Atypes having` pin connections, is fastened by boltstoY .the bottom of well 14,

Anly th'e'ilarnent power and .direct-current connecf tions to tube 12 connect to'thepins of socket 19. Tube 12 lisrirlsulated direct-current wise from ground-plane in order to utilize a cathode-biasing resistor 21 which connects between ground-plane 10 andisocket-pin 22 that is connected to the tube cathode. However, in the case of some tubes, such as the 416B, which have a bypass capacitor built into them, well 14 may be rnade of conducting material to thus connect cathode surface 1,8 to ground. Many tubes do not have a built-in bypass capacitor, and the support provided here `for tube 12 vtuev be used in either case.

` Contact fingers 17 havepa relatively large area spaced closely to ground-plane 10 to provide a virtually zero impedance between the cathode and ground at the ultrahigh frequencies pertainingy to the invention to thus ground the cathode of tube 12. A pair` of pins 2,4 and 26 connect to a filament voltage supply to provide the necessary heater voltage.

The radio-frequency input to the amplifier is received` at a coaxial coupler 27, which has its outer conductor 28 fastened to ground-plane 10; and an inner conductor 29 is supported therein by a dielectric spacer- 31. An antenna 32 might, for example, be connected by arcoaxial cable to coupler 27.

A mismatching transmission line 33 'connects between coaxial coupler 28 and the input `of grounded-cathode tube 12. Mismatching line 33 is comprised of ground,- planc 10 and a single at strip 34 of conducting material with a spacing between them that obtains a required oharaotofistio impedance as will oo. explained. boiow- 'lfhe inner conductor 29 o f coaxial connector 27 connoois io Strip 34. ai. a'. Soosioiiiiol dioiilnoo from eitherof. its oodo- Ooo. ood. oi ,Sirio 34, is, iiXod to o. Split-,fina 3o, W-iiioli oogogos tho oootrolrafiil-f-oooiaotiog .Soif oef oi?v tube 12, and thereby connects, radio-frequency `rivisti-liefl tween the grounded lcathode and the control-grid Aott tube Tho oooosiio oaooi strip @4Lis,eroeoaooeoiiisollio 75 stub 37 which has `a metallic block 38 received as a sh0rtcircuit between strip 34 and ground-plane 10. Block 38 may be slideable to provide an adjustable reactance.

A second reactance-cancelling stub 41 is provided at the tube end of line 33 and comprises a metallic strip 42, that also is xed at one end to grid-contact ring 36, and a shorting block 43 between its other end and ground- Plano 10- Block 4.3 may bo kSlideablo to provide a reactance adjustment for stub 41. Hence, stub 41 may be used to cancel the susceptance at the input of tube 1,2., which is between its grid and cathode. The susceptance cancellation by stubs 33 and 41 is used to obtain required mismatching conditions, which will be explained below.

A shield 44, which has an E-shaped cross-section, may surround ungrounded strips 34 and 42 to prevent stray coupling. A neutralizing stub 46 is comprised of rods 47 and 48 shown in Figure l. Upper rod 47 connects at one end to a contact ring 49, which engages the plate-contacting surface 51 of tube `12; and lower rod 48 connects at one end to grid-contact ring 36, as shown in Figure 2. A radio-frequency shorting bar 52 is slideably received over rods 47 and 48, and a thin dielectric spacer 53, separatesl opposite parts of shorting bar 52 to provide direct-current insulation between rods 47 and 48V, although the capacitance across dielectric spacer 53 provides al'virtual short circuit at ultra-high frequencies. Stub 46 is adjusted to resonate with the grid-to-plate interelectrode capacitance of tube 12 and, therefore, neuiriilizos this Capacitance by making the grid-to-plate impedali@ appear as o very large. resistance- A Soo9i1 l tubo 61 is provided, Whioh might be a type of triode eleetron tube generally known as a pencil triode.` Tubey 61 has; an intermediate-flange member 62 which connects to the tubes control grid, a cylindricalplate-contacting surface 63 at its lower end in Figure 2, and a c ylindrical-cathpde-contacting surface 64 at its upper end in Figure 2. Filament leads extend from the end of cathode-contact surface 64. The grid of tube 61 is biased by means of a voltage divider comprising res istors 60 and 6 5, with a lead connecting their inter mediate point to grid flange 62.

An annular dielectric member 66 is received between ground-,plane 10 and grid-contacting surface 62; and they may be cemented together to support pencil triode 61 in an insulated manner from ground-plane 10. Therefore, .they control grid will not be at ground-potential direct-current wise; although the capacitance between grid flange 62 and ground-plane 1() is large enough to provide virtuallyzero conductance betweenthe ground-plane and grid at the ultra-high frequencies used with this` inventin. As a result, grid 62 is grounded radio-frequency wise, andtube 61 operates as a grounded-grid stage.

Av transmission line 67 includes a metallic strip 68 that connectsbetlween the plate of planar tube 12 and the cathode of pencil triode 61. The other side of line 67 is primarily the grounded-surface of an E-shaped shield 70. Metallic strip 68 connects at one end to plate-contact ring 49, which engages the plate of planar tube 12, and connects at its other end to a cathode-contact ring 69, which engages cathode-contacting surface 64 of pencil triode 61. Also, strip 68 connects the plate of tube 12 in series with the B-plus supply voltage through the directcurrent plate impedance of t'ube 61.

. A reactance-cancelling stub is provided at each end of transmission line 67. One stub 71 is provided by a strip 72 which connects at one end to a plate-contact ring 50;

and an adjustable shorting block 73 is received betweeni strip 72 and ground-plane 10. Contact ring Si) is split on one side and is insulated direct-current wise from plate,- surface 5 1 by anY insulating ring 55, although the capacify foooo oorooo .fioeSS Provides a Sliort oirouif io F currents.

In a similar manner, another stub 76 is provided by an- .other sitio 71 Wllioli is ooaoooiool ai oaooiiil toa ooilioiof contact ring 78; and another shorting block 79 is adjustably received between strip 77 and ground-plane 10. Contacting ring 78 is split on one side and is insulated direct-current wise from cathode-surface 64 by an insulating ring 80, although the capacitance across ring 80 provides a short circuit to U. H. F. currents.

Stubs 71 and 76 are surrounded by E-shaped shield 70 to prevent stray coupling.

Stub 71 is used to cancel the net susceptance at one end of transmission line 67, and the other stub 76 is used to cancel the net susceptance at the other end of transmission line 67, in order to provide a required set of mismatch conditions, as will be explained in detail below.

The output of second tube 61 is removed by means of still another strip-type transmission line 81 that includes the ground-plane and a strip 82, which connects at one end to an annular contact ring 83 received about the platecontacting surface 63 of pencil triode 6l. An output coaxial connector 84 has an outer conductor 86 fastened to ground-plane and an inner conductor 87 fastened to strip 82. Connector 84 receives an outgoing coaxial transmission line (not shown) which might connect to a following amplitier or frequency mixer stage. A capacitor 90 is serially connected to inner conductor 87 and blocks direct-current but provides almost no impedance to ultrahigh frequency currents.

A shorting slug 88 is received between the opposite end of strip 82 and ground-plane 10 to make the portion of the line between connector 84 and slug 88 a reactancecontrolling stub 89 for matching the output of the amplirier to a load. Slug S8 has a thin dielectric portion 91 that insulates strip 82 from ground-plane 10 direct-current wise wnile permitting a virtual short circuit at ultra-high frequencies. A shield 85 surrounds strip 82.

It is necessary to understand some basic amplifier noise theory in order to understand this invention. The noisefigure of a given amplifier is primarily dependent upon the equivalent noise resistance, Req, of the amplifier, the input transit time loading conductance, Gt, of the amplifier, the input loss conductance G1 of the amplifier, and` the source impedance, Ga, connected to the amplifier input.

An equivalent noise resistance, Req, is a tool for evaluating the noise generated in a given amplifier, and may be defined with respect to an ideal noiseless amplifier, having the same gain as the given amplifier, as: that value of resistance which must be connected to the input of the ideal amplifier to provide an output noise current that is equal to the noise current generated in the output of the given amplifier.

As stated above, the best noise-figure for an amplifier is not generally obtained with a matched condition between the input impedance of the amplifier and the source impedance. On the other hand, the noise-figure may be best with a particular unmatched input conductance ratio G/G,n and may be determined by the minimum point on a curve in Figure 3, such as one of curves 91, 92 and 93 or a curve (not shown) which is between them. In Figure 3, the ratio Ga/Gt assumes that G1 is negligibly small at the ultra-high frequencies involved herein. It is emphasized that only for ultra-high frequency groundedcathode ampliers is the optimum ratio Ga/G, equal very nearly to the optimum mismatch ratio Ga/Gm at the input to the amplifier tube.

The product, GtReq, is a parameter fixed by the choice of a given amplier tube and is important in evaluating amplier tubes for use in low noise applications. it is noted that the optimum noise-figure (which is the minimum point on each of the curves) improves as the product, GtReq, becomes smaller. And it is further noted that the optimum noise-figure becomes better as the required mismatch becomes greater.

Although the noise-gure of the amplifier depends pri- 'marily on the product. GtReq, and the input mismatch, 'the selectivity of the amplifier depends primarily on its input conductance Gm. In the case of the groundedcathode amplifier, the input conductance, Gm, is approximately equal to the transit-time conductance, Gt. However, in the case of the grounded-grid amplifier, the input conductance, Gm, is larger and is equal to gn Gdr-rn where gm is the mutual conductance of the tube, Rp is the plate load source resistance, and rp is the plate resistance of the tube, all at the given operating frequency.

Thus, where a low-noise narrow-band amplifier is required, not only should the chosen tube have a minimum product, GtReq; but the value of Gt should also be minimum, in a choice between tubes having the same product but different values of input conductance.

In the U. H. F. band, the optimum noise-ligure, Fong, may be approximately determined (when the product, GtReq, is much less than one) by the formula:

Foot. z1'i2\/5RQG And the optimum conductance ratio Gg 0pt.

for best noise-figure may be determined approximately (when the product, ReqGg is much less than one) by the formula:

(Goma/23% (s) wherein (Gamm is that source conductance which must appear at the amplifier tube input to provide the best noise-ligure.

It can further be shown that the transit time loading conductance Gt of a tube increases approximately with the square of the frequency; and the optimum noise-figure decreases accordingly with frequency.

This mismatching requirements of this invention are obtained in proportioning its various transmission line components. It will be realized that problems occurring in providing mismatched conditions with .a distributedconstant circuit are substantially different from those occurring in lump-constant circuits; and this invention shows how the mismatched conditions can be obtained using distributed-constant circuits.

A theory for mismatching the terminals of a distributedconstant transmission line may be stated as follows: Where opposite ends of a transmission line are terminated in unequal and unmatched conductances, wherein there is Zero susceptance at both ends of the line, the ratio of connected sending end conductance to sending end source conductance is equal to the ration of connected receiving end conductance to receiving end source conductance.

The theory may be explained in more detail using Figure 4, which shows a section of transmission line having a characteristic admittance, Y0. Sending-end conductances are taken along a plane s, situated at the sendingend terminals; and receiving-end conductances are taken along another plane r, situated at the receiving-end terminals. Also, the net susceptances at both the receiving and sending ends are zero; and the end admittances are, therefore, pure conductances, which are taken at each plane in vopposite directions along the line.

Conductances Gsr and GSS are taken at plane s; where Gsr is the sending-end source conductance, looking at the load end of the line; and GSS is the sending-end conductance, looking at a sending end connected impedance,

Conductances; Cirr and Gre are taken'atplane r; where Gr, fis the ,load-.endffconducta-nce; looking' at a load-:end connected?impedance,` and GTS-fist-he load-end sourceconductancmflojoking; toward the ,sendingendY off-fthe ;line. These actual andwsource conductancesare v:related-.by,the above theorem as given in the following expression:

' Furthermore, theconductancesappearing across planes stand' rfarerelatedto thechfaracteristicadmittance, Y0, (reciprocal-ofcharacteristic f impedance)l ofy lineby the Afollowingformula:

As a result, the susceptance connected at each end of "theilineis cancelled Vto cause-a net susceptance of zero 'at' each end of'theline. iFurthermore; each of the products GYSGss and GWG,-r must be greater" thanleither Bssl or BTT, in order -for the characteristic admittance Y tofhave a real value.

The length ofthe transmission line is related to the characteristic admittance as follows:

where L is the length of the transmission line between the receiving and sending' terminals, and X is"the wavelength in the'line of propagated energy at the chosen frequency. Consequently, it will be desirable, in some casesv to make connector 27 s'lideable to provide linel with an adjustable length.

In'regard to the inventionshown inFigures 1 and' 2, the

lrst, adjustment is of neutralizing stub 46, which is resovnated with the grid-to-plate capacitance of tube 12 at a desired operatingfrequency forjthe amplienwhich might be 1000 megacycles, for example.

In regard to iirst transmission line 33, in Figures l and 2, stubs 37 and 41 arelthen adjusted so that there is zero 1 net susceptance at opposite ends of'line 33 at the amplifier operating frequency. This is done by positioning slugs 38 and 43 along their respective stubs. Thus, the input susceptance of tube 12 is cancelled, and any susceptance -provided at connector 27 ,dueto its connection to a leadin linesuch as one connectedl to an antenna 32, is cancelled. Hence, pure conductances yresult at' opposite ends of transmission line 33'.

It follows'then with transmission line`33 that: Gfrr is actually the input conductance, Gin, of tube 12; and Grs Ais the apparent-source conductance,` Ga, at the input to tube 12.

vAfter Vthe selection of a particular tube llZ is determined by `the product, Gtleq, 'the optimum mismatch ratio "Ga/G', may be' determincd'v/ith'theuse of Figure 3, and thefollowing'may be stated:

Mismatch Ratio l (9) The dimensions Afor-transmission line 33 and its con.- nectingstubs 37-.and41 may be determined fromthe value of characteristic admittance found using `Formula 5 above.

Thef use of;A ystrip-type ltransmission lli'nepermits the .characteristic admittance,0, to` be controlled' and varied with relative ease. The value of'Y-may be controlled by adjustipgthemidth'of thesingle ungrounded strip, or by adjusting'the spacing between itand theground-plane, or.by.-providir1g--and` controllingthe dielectric material betweennthestrip andthe,groundylplane. `In any case, nal adjustment of',characteristic-admittance can easily be made by adjustingathe spacing between',- the strippandlhe groundfplane.

The stubs can convenientlythave-thesarne characteristic -irnpedanceas -the-.transmission-line to which they connect.

Transmission I ine- 67, which connects;tl1e.o!.1tputz-of grounded-cathodetube .12 `to the ,input Lof grounded-grid tube. 61, .may-,beproportionedv using the sameprinciples-as describedabovegfror. .inst-.transmission line 33.

Stubs 71f`andn dxareprovided at-the oppositef-endsof second line, 67I :to provide source. susceptances thatcancel the `susceptanbes `'whichmay be` provided :bythe tubes. Hence, ythe mismatching theorem stated -aboyegapplies .Ther.mismatchingrimpedance ratiovfor: second; transmission line.. 51 may ,be determined-by 'the value; on mismatch required `at the input to grounded-grid tubeg61-ito provideit .with aia-optimumnoise-iigure. Obtaining the proper..rr'lisznatchA .forgseconm tube-l-isY notv as` critical as `obtaininga; properL vmismatch-forl first tubel 12,; rsince the former .hasfmuch less=eiectuponthe overallnoise igureof theJtwo-stage; amplier. ,Also, the. choiceV of mismatch" ratio; foi-@second transmission-.line 67 is, inuencedby stability considerations for-firsttubell Generally speaking, the, stability-of -the ',iirstf tube increases asv the4 ratioof its..plate,impedance opiate-load impedance increases,i..e., -`as its apparent-plate-load.impedance becomesfsmller. p A A p Therefore, armismatehing ,ratiov GST/Gsf is required: whereinEGSVS isthe Vl'olate conductance. looking into-the tube; and Gsr 'is-'gtheplate-loadconductance, which is determined by looking .from the, plate :intotransmission line 67.

The considerations for determining the input-mismatch- Ving ratio Ga/.Gmto ag-rounded-grid amplier are notthe same` as'fthose `used for determining the input-mismatching ratioof @grounded-cathode.amplilier because of thedifference in' .input conductance Gm. Figure'3 can also be used to'deter inefthe optimum mismatching ratio- Gm/Gm for av gronded-gridf4 amplierbut lthe procedureisless direct 'thanfor'the grounded-cathode amplier. Thus,

.the -ratio (gt) 2G. spi.

dete'rt'riin'df'byfEquationfBf above,` 'and which is approximately' thel "input rn'is'zna't`,cl 1"`"ratiofjof`a grounded-cathode amplifier, 'canonly' 'indirectly' 'provide' the optimum mismatch ratio*'ora'grounded-grid'amplier. This may be donei'withthe folldwingfo'rmula:

astucia ratio required at the input to grounded-grid tube 61 for best noise-gure provides an optimum-plate impedance ratio for the grounded-cathode tube 12 to provide the best compromise of stability and gain.

Consequently, Formulas 4, and 9 may be applied when the desired mismatch ratio for the particular tubes is found. Thus, the characteristic admittance of second transmission line 67 may be calculated using Formula 5 above, and the size of strip 68 and its spacing from the top of grounded shield 70 determined accordingly. A final adjustment of characteristic admittance may be obtained by adjusting the spacing between strip 68 and shield 70.

The third transmission line 81 is used to connect the output of grounded-grid tube 61 to an ultimate load, which might, for example, be a mixer stage. Generally, maximum power considerations will prevail in determining the transmission-line characteristics of line 81, since the amplifier will often provide sufficient gain to make the noises generated in the following stages negligible. Where, however, the noise considerations of the following stage are very important, which may be important with certain types of mixers, then transmission line 81 should be mismatched in the same manner as previously discussed.

The output impedance at connector 86 may be controlled over a wide range by making connector 86 slideable along the length of transmission line 82. Thus, an impedance match may then be obtained between connector 86 and an outgoing transmission line (not shown) by adjusting the length of line 82 and the position of slug 83.

Stub 89 is used to control the susceptance which may exist in the amplifier output.

It is, therefore, apparent that this invention provides an amplifier which obtains a very low noise-figure in the ultra-high frequency region. Furthermore, it can be seen that this invention also provides a U. H. F. amplifier which is capable of having a high input impedance, and, accordingly, is capable of having a narrow-band input. It is further noted that the adjustments required in the invention are relatively simple, mechanically speaking, and have a given order which permits variable tuning over a range of ultra-high frequencies with good stability.

While a particular form of the invention has been shown and described, it will be obvious to a person skilled in this particular art after studying this specification, that this invention is capable of many modifications. Many changes, therefore, in the construction and arrangement of this invention may be made without departing from its full scope as given bythe appended claims.

What is claimed is:

1. A stable low-noise U. H. F. amplifier capable of having narrow-band selectivity comprising a fia-t ground-plane member of conducting material, a well-type tube support fastened to said ground member, a first electron tube supported in said well, the grid and plate of said first tube extending through an opening formed through said ground member, a coaxial input connector with its outer conductor connected to said ground member on the same side as said well and having its inner conductor passing through an opening formed in said ground member, a flat metallic conductor connected at one end to the grid of said first tube and connected at an intermediate point to the inner conductor of said input connector, a first metallic slug received between said ground member and the portion of said first metallic strip extending beyond said input connector oppositely from said first tube, a second metallic strip connected at one end to the grid of said firs-t tube, a second metallic slug received between said second metallic strip and said ground-plane, a second electron tube extending through an opening formed in said` ground member and having its grid connected radio-frequcncy-wise to said ground member with its cathode extending on the same side of said ground member as the plate of said first 10 tube, a third metallic strip connected between the pla-te of said first tube and the cathode of said second tube, a fourth metallic strip connected at one end to the plate of said first tube, a third slug having low radio-frequency im pedance supported between said fourth metallic strip and said ground member, a fifth metallic strip connected at one end to the cathode of said second tube, a fourth slug having low radio-frequency impedance connected between said fifth metallic strip and said ground member, an output coaxial connector having its outer conductor connected to said ground member with its inner conductor passing through an opening in said ground member, a sixth metallic strip connected at one end to the plate of said second tube and connected at an intermediate point to ythe inner conductor of said output connector, a fifth slug of conducting material received between said ground member and said sixth metallic strip on its porti-on extending beyond the output connector oppositely from said second tube, a neutralizing stub connected radio-frequency-wise between the plate and grid of said first tube to neutralize its plate-to-grid interelectrode capacitance, whereby the characteristic impedances and lengths of the strip-type transmission lines can be controlled to provide a minimum noise ligure for the amplifier.

2. An amplifier as in claim 1 in which said fourth metallic strip is insulated direct-current-wise but is connected U. H. F.wise from the plate of said first tube, said filth metallic strip is insulated direct-current-wise but is connected U. H. F.wise from the cathode of said second tube, said fifth slug providing direct-current insulation but not U. H. F. insulation between said sixth strip and said ground member, direct-current blocking means are incorporated into said neutralizing stub without substantially affecting U. H. F. conduction, the grid of said second tube is insula-ted direct-current-wise from said ground member but connected U. H. F.wise, a plate-voltage supply is connected to said sixth metallic strip, a voltage divider is connected between said plate-voltage supply and said ground member, an intermediate point on said divider is connected to the grid of said second tube to properly bias it, and a resistor is connected between said ground member and the cathode of said first tube to properly bias i-t.

3. A stable low-noise U. H. F. amplifier comprising a ground-potential member, an electron tube supported by said ground-potential member, an input transmission line connected at one end to the grid and cathode of said tube, a pair of susceptance-cancelling stubs connected respectively to the opposite ends of said input transmission line, `a load connected between said ground potential member and the plate of said tube, the ratio G; opt.

being the optimum mismatching ratio for said tube, and the length (L) of said input line being approximately determined by the expression where Y0 is the characteristic impedance of said input line, Gsr is the conductance of said input transmission line, and Gt is the input conductance of said tube, and 7\ is a chosen U. H. F. wavelength.

4. A U. H. F. amplifier as defined by claim 3 in which said input transmission line, second transmission line, and stubs each comprise, a strip of conducting material supported substantially parallel to said ground-potential memer. i

5. A stable low-noise U. H. F. amplifier comprising a ground-potential member, first and second electron tubes supported by said ground-potential member, said first tube connected in a grounded-cathode manner, and said second tube connected in a grounded-grid manner, an input transmission line connected at one -end to the grid and cathodeofsaid, first tube, a, pair of susceptance-cancelling stubs connected `resp:activelytol the opposite ends ofsaid input line, a second transmission linet connectedv atone endito the plate andwcathode o f said first tube, and connectedv @the .other ad taille smaad-potential member and the cathode ojfusaid second tube, a second pair ofsusceptancecancelling stubs connected respectively to the opposite nds Off saisi. s sgad. traftsnfussionA line? the ratig Gini opt.

beingthe optimummismatching ratio for'said first tube, and the ratio Gina opt.

beingthe optimum mismatching ratio for said second tube, iiI iein Gm isthe input conductance of either tube andV s the .source conductance at the` tube input,- yandthe length oteafchot,saidztransmission lines being approximately-determined byhexarsssiom lG y [Gatinais W]- Where .Yois .the chalvasteristctadmittancgot malins. and

an electronwdischarge tube supportedpby said `groundpotential lmember and havingits cathode. vcmnectedV radiofrequency-wise tov saidL member,` an input transmission line,

comprising said, ground member,V and a. co nducting strip fastened at one end to the` grid ofksaidhfirst tube, a pair of susceptance-controlling stub means uconnected respectively toutheends of said input linefforcancelling the net ssceptanceat oppositel ends ofsaid inputline, said input, line having its chalateristic, admittance Y() definedv where G1.S is the source conductance at the input to said tube, Gss is the conductance connected at the sending :end ot, said input line Bfr isthegsusceptancepof.the

stub vconinecsted, to the tube .end `of saidinput line, GS, is` the conductance at the input tosaidinput line,Gn, is the where Gris thetran'sit-time loading conductance of said tube,` and Reqwis the equivalent noise.resistance of said tube,l a neutralizing st ub connected between the gridl andplateof said'tube to resonate with its grid-to-plate n interelectrodeV capacity, and the output of said tube remgratbetwesn itsplateandtsaid gaand-Patenti@ memfifi...

fagsnsaas for. prgvidinga gnductance atthereceiving end of said line, with conductance Grp provided by the receiving end, and conductance Grscon-l nected to the receiving end, comprising a ground member, a strip of metallic material supported from saidA ground..- member and insulated therefrom, aiirst stubconnected to the sending end of said transmission line, a` second, stub connected to the receiving end of said line, shortingmeans with eachof said stubs to cancel the susceptance. at Opposite ends of said transmission line, the susceptance.. detined by the expression:

Y0 Cot where-Y0 is the characteristic admittance of said line,A is the propagation constantof said line, and L is the length of said line between said stubs, and the length of said line being approximately defined by the.quantity:

where Gsn is the conductance. connected to the sending endrof said line.

9. Means for providing at thel sending and receiving terminals of a transmission linea required conductance mismatch which may be defined by the expression:

conductance mismatch ratio=G,s/G.=Gs/GS8 wherein Grr is the conductance connected to the receiving end of the line, Grsisgthe line conductance provided at thereceiving endGss is the, conductance connected toL thevsending endmofpthe line, Gsr is the line conductance provided at the. sending end, comprising a first segment.. of transmission line connected to said sending terminals, asecond segment of transmission line connected tol said receiving terminals, respective short-circuiting members,v receivedin each. of said segments to provide a cancellingsusceptance BL at opposite ends of said transmission line. approximately defined by the expression;

where Y0 is the characteristic admittance of said transmission line, is the propagation constant of said line,v L is the length of said line, and said characteristic ad. mittance is further approximately dened as;

10. Means for providing a required conductance miS.-vr match at opposite terminals of a strip-type transmission linel utilized in ultra-high-frequency low-noise amplif-y iiers, comprising a ground-potential member, a tlat metallic-conducting strip-supportedy substantially parallel to said ground member and insulated therefrom, said strip'. providing said transmission line with a characteristic admittance Y0 and alength- L, a second strip of conducting materialrconnected at one end to an, end of said .first strip and spacedk similarly from said ground member, first` slug Ameans,receivedbetween saidA secondv strip andA said; ground member to provide a large -U, H. F. conductancev betWeen,said-,second strip and saidnground member, aithird flat-.metallic strip havingy one end connected to the-op.` posite end` of saidv first strip and similarly spaced froml said groundmember, second slug means supported be-y tweensaid third strip and said ground member to provide a large U. H. F. conductance between said third strip andsaid `ground member, each of said slug means posi-,- tioned to. provide substantially zero net susceptance at said oppositel terminals of said transmission line, and the characteristic impedance Y0, and length. Lof said. transmission line being approximately deiined by:

the respective line and terminalYA conductances \at,,one e11dt,. of Vthe line, Gsr and` GSB; are the, respective ,linetfandn terminal. .sgndutanses atvv the 4oizpositeend sot. .that-linearA asumo and the respective conductance mismatches at opposite ends of the line are defined as:

d, a, ZMGT.

l1. A stable low-noise U. H. F. amplifier comprising a ground-plane member, a pair of electron tubes supported by said ground-plane member and each including a cathode, a grid and a plate, the first tube having its cathode connected ultra-high-frequency-wise to said ground-plane, the second tube having its grid connected ultra-high-frequency-wise to said ground-plane, a neutralizing stub connected between the grid and plate of said first tube, first conducting means connected at one end to the grid of said iirst tube, an input-transmission line being comprised of said rst conducting means and said ground-plane member, a first pair of stubs connected respectively to the opposite ends of said input-transmission line, said stubs cancelling the reactance at opposite ends of said input line, a second conducting means connected between the plate of said first tube and the cathode of said second tube, an interniediate-transmission line being comprised of said second conducting means and said ground-plane, a second pair of stubs connected respectively to opposite ends of said intermediate line, said second pair of stubs cancelling the net reactance at opposite ends of said intermediate line, the characteristic conductance Y0, and length L of each of said lines being related to the conductances Gsr and Grrs looking into the opposite ends of the line, and the respective conductances Gss and Grrr connected to the respective ends of the line at an operating wavelength )t as follows:

the ratio G,.s/G1TT presented to each of said tubes being approximately their optimum mismatch ratio, and the output of said amplifier being provided from the plate of said second tube.

12. A stable low-noise U. H. F. amplifier capable of having narrow-band selectivity comprising a ground-potential member, a first electron tube supported by said ground member with its cathode connected radio-frequency-Wise to said ground member, a rst flat metallic transmission strip supported substantially parallel to said ground-member and connected at one end tothe grid of said iirst tube, an input transmission line being comprised of said first transmission strip and said ground member, a first stub connected to one end of said input transmission line to provide a source susceptance at the opposite end of said input line that substantially cancels the net susceptance at the opposite end, a second stub connected to the opposite end of said input transmission line to provide a source susceptance at the rst end that substantially cancels the net susceptance at the first end, a second electron tube supported by said ground-member with its grid connected radio-frequency-wise to said ground member, a second at metallic strip supported substantially parallel to said ground member and connected at one end to the plate of said lirst tube and connected at its other end to the cathode of said second tube, a second transmission line comprised of said second strip and said ground member, a third stub connected between the ground member and plate of said first tube to substantially cancel the net susceptance at the input to said second tube, a fourth stub connected between the cathode of said second tube and the ground member to substantially cancel the net susceptance at the output of said lirst tube, a third dat metallic strip supported substantially parallel to said ground member and con- 70 2,615,998

ected at one end to the plate of said second tube, an

output transmission line being provided by said ground-` plane and said third strip, a neutralizing stub connected radio-frequency-wise between the grid and plate of said rst tube to neutralize its grid-to-plate interelectrode capacitance, the characteristic admittance Y0 of each ot said lines being dependent upon the widths of said strips and the dielectric-spacing between them and said ground member,

al .al (geen) where G51. is the conductance looking into the input of the respective line, and A is the operating wavelength.

13. An amplifier as defined in claim 7, and including a second electron tube supported by said groundpotential member, the grid of said second tube connected radio-frequency-wise to said ground-potential member, an intermediate-transmission line, comprising said groundpotential member, and a second conducting strip connected at one end to the plate of said iirst tube and connected at its other end to the cathode of said second tube, a second pair of susceptance-controlling stub means connected respectively to the opposite ends of said intermediate-transmission line to substantially cancel the net susceptance at opposite ends of said intermediate line, said intermediate-transmission line approximately satisfying the formula:

stub connected to the input to the second tube, s, is the conductance at the input to said intermediate line,

Gfrr is the input conductance of said second tube, and BSS is the susceptance of the stub connected to the output of said first tube, and the optimum input-impedance ratio of said second tube being where Rp is the load resistance of said second tube, rp is its platey resistance, gm is its transconductance, Gt is its transit-time loading conductance, and Req is the equivalent noise resistance of the second tube.

@than References Cited in the file of this patent UNITED STATES PATENTS 2,143,671 Zottu Ian. 10, 1939 2,419,985 Brown May 6, 1947 2,524,821 Montgomery Oct. 10, 1950 Charchian Oct. 28, 1952 

